Astronomy 2e · Cratered Worlds
General Properties of the Moon
On this page 9 sections
In 30 seconds
The Moon is Earth's nearest neighbor — about 384,000 km away, roughly 30 Earth-diameters — and the only other world humans have walked on. It is also a very strange satellite: relative to its planet, the Moon is enormous, with a diameter about one-quarter of Earth's (~3,500 km vs ~12,700 km) and a mass about 1/81 of Earth's. Jupiter's largest moons, by contrast, are tiny specks next to their planet. Earth and the Moon are almost a double planet — a clue to how the Moon formed (Topic 4).
Size is only the beginning. The Moon's average density Mass per unit volume (g/cm³). Full entry → is ~3.3 g/cm³, much lower than Earth's ~5.5 g/cm³, telling us it has little or no large iron core. Its surface gravity is about one-sixth of Earth's, and its escape velocity The speed needed to escape a body's gravity permanently. Full entry → is so low that it cannot hold an atmosphere — no air, no weather, no sound, no blue sky. It rotates exactly once per orbit, so the same face (the near side) always points toward Earth. Apollo-era seismometers revealed a quiet interior: crust, mantle, and at most a small core, with only faint moonquakes. These properties are the foundation for the rest of the chapter.
Why this matters
- The one world we've walked on: direct measurements (samples, seismometers, laser reflectors) calibrate how we interpret other worlds.
- Tides and cycles: the Moon's gravity drives ocean tides and has governed timekeeping for millennia.
- Formation science: the Moon's low density and odd size are evidence for the giant-impact hypothesis.
- Future exploration: plans to return humans to the Moon and to use polar water ice depend on knowing its basic physics.
The college version
Core Concepts
A big moon around a small planet
The most distinctive fact about the Moon is its size relative to Earth. A quarter of Earth's diameter and 1/81 of its mass may not sound like much, but compare with other planets: Jupiter is 300 times Earth's mass, yet its largest moon is only ~1/10,000 of Jupiter's mass. The Earth–Moon system is so balanced that the two bodies orbit a common center of mass (the barycenter The common center of mass about which two bodies orbit. Full entry →) that lies inside Earth, just below the surface. This unusual ratio is a key clue that the Moon did not form like ordinary moons — it looks like the outcome of a giant collision (Topic 4).
Density and composition: the missing iron
Density is mass divided by volume, and the Moon's average density (~3.3 g/cm³) is much lower than Earth's (~5.5 g/cm³), whose high value comes from a large iron-nickel core. The Moon therefore cannot have a big iron core — its composition is closer to Earth's rocky mantle. Combined with near-identical oxygen-isotope ratios in lunar and terrestrial rocks, this points to a common origin. The Moon is essentially all rock: a thin crust, a thick mantle, and at most a small (or absent) iron core.
Gravity, escape velocity, and the missing atmosphere
Surface gravity on the Moon is about 1/6 of Earth's — a 100 kg astronaut would weigh about as much as a 17 kg object does on Earth. The Apollo footage shows it: astronauts bounded in slow, high jumps because their bodies were tuned for six times the weight.
Low gravity has a deeper consequence: escape velocity. To leave the Moon forever, a molecule needs only about 2.4 km/s (compared with 11.2 km/s on Earth). Gas molecules at lunar surface temperatures routinely reach such speeds, so any atmosphere the Moon once had leaked away long ago. What remains is an exosphere An extremely thin outer atmosphere, closer to a vacuum than to air. Full entry → — a vanishingly thin haze of atoms (sodium, potassium, helium from the solar wind), closer to a vacuum than to air. With no atmosphere: the sky is black even at noon, no wind or weather, no sound, and surface temperatures swing from about +130 °C in sunlight to about −180 °C at night (commonly taught values).
Synchronous rotation: the same face, always
The Moon rotates on its axis, but its rotation period equals its orbital period (~27.3 days), so one lunar day lasts one lunar orbit. This is synchronous rotation Rotation period equals orbital period, so the same face points toward the planet. Full entry → (tidal locking Gravity brakes a body's spin until it rotates synchronously. Full entry →): Earth's gravity braked the Moon's spin over billions of years until the near side always faced Earth. The far side was unknown until 1959, when the Soviet Luna 3 spacecraft photographed it. A common mistake is to call the far side "the dark side" — it gets as much sunlight as the near side, just facing away from us.
Interior and activity: a quiet world
Apollo astronauts left seismometers on the Moon that operated for years. The data revealed a layered interior — crust, mantle, and a small (or possibly absent) core — and showed moonquakes are real but tiny, mostly triggered by tides and meteorite impacts. There is no plate tectonics and no active volcanism; the last major eruptions ended more than a billion years ago. The Moon also has no global magnetic field today, though magnetized rocks show it once did when its core was molten. In short, the Moon is a geologically dead world — which is precisely why its surface preserves a record of the solar system's violent past.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| The far side of the Moon | The dark side of the Moon | The far side faces away from Earth, not away from the Sun — it gets full daylight on a regular schedule. |
| "The Moon doesn't rotate" | Synchronous rotation | The Moon rotates once per orbit; that's why we always see the same face. |
| Weight | Mass | Mass is the amount of matter (unchanged on the Moon); weight is the gravitational force on it — one-sixth as much there. |
| The Moon's gravity causes tides | It is the only cause | The Sun also raises tides; the Moon's influence is the larger one (see Chapter 4). |
| The Moon "shines" | The Moon emits light | It reflects sunlight — moonlight is just sunlight bounced off rock. |
| The Moon's density being low | The Moon being less rocky | It is all rock; Earth is abnormally dense because of its iron core. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The Moon is Earth's big, rocky companion — big for a moon, but much smaller and lighter. Its gravity is so weak you'd weigh only about one-sixth as much there and could jump really high — but it's also too weak to hold air, so there's no weather, no sound, and a black sky even in daytime. The Moon spins once every time it goes around Earth, which is why we always see the same side. It's a quiet, cold, airless place that has kept every scar.
Worked example
Walk through a classic argument (no math needed). At lunar temperatures, a nitrogen or oxygen molecule moves at roughly a kilometer per second — fast, but well below Earth's 11.2 km/s escape velocity, which is why Earth keeps its air. Move the same molecule to the Moon: escape velocity is only ~2.4 km/s, and the fastest molecules routinely exceed it. Each time one does, it sails into space forever; over hundreds of millions of years, the leakage removes every trace of gas — heavier molecules linger longer, so the Moon's faint exosphere skews toward sodium and potassium. Add the Sun: by day the surface heats above 100 °C, giving molecules more speed, and the solar wind can sweep atoms away. No atmosphere → no insulation → savage temperature swings → no protection from micrometeorites and radiation. That single number — escape velocity — explains most of what makes the Moon unlike Earth, and it is the argument astronomers use for exoplanets.
Key takeaways
- Scale: distance ~384,000 km; diameter ~¼ of Earth's; mass ~1/81 of Earth's (commonly taught reference values).
- Density ~3.3 g/cm³ vs Earth's ~5.5 g/cm³ → the Moon has little or no large iron core.
- Surface gravity ~1/6 g; escape velocity ~2.4 km/s (vs 11.2 km/s) → no atmosphere (only an exosphere): black sky, no weather or sound, temperature swings from ~+130 °C to ~−180 °C.
- Synchronous rotation: rotation period = orbital period (~27.3 days) → same face (near side) always toward Earth; the far side is not "dark."
- Interior: crust, mantle, small (possibly absent) core; Apollo seismometers recorded only weak moonquakes.
- Geologically dead: no plate tectonics, no active volcanism, no global magnetic field — the surface is an ancient record.
- The Earth–Moon size ratio is unusually large → key evidence for the giant-impact origin (Topic 4).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why does the Moon have no atmosphere, and what thin remnant does it actually have?
Show answer
The Moon's low escape velocity (~2.4 km/s) lets gas molecules escape into space, so any atmosphere leaked away long ago. What remains is an exosphere — a thin haze of atoms like sodium, potassium, and solar-wind helium.
What does the Moon's low average density (~3.3 g/cm³) tell us about its interior?
Show answer
Low density means no large iron core (Earth's high density comes from its core); the Moon is mostly rocky mantle-like material, consistent with a giant-impact origin.
Explain synchronous rotation: why do we always see the same side of the Moon?
Show answer
Synchronous rotation means the Moon's rotation period equals its orbital period (~27.3 days). Earth's gravity tidally locked the Moon, so the same hemisphere always faces us.
How much would a 60 kg student weigh on the Moon, and why is "weigh" the right word?
Show answer
About 10 kg of Earth-weight (60 ÷ 6). Weight is the gravitational force on a mass; the student's mass is unchanged, but lunar gravity is 1/6 as strong.
What evidence shows that the Moon is geologically inactive today?
Show answer
Apollo seismometers recorded only weak moonquakes, there is no active volcanism (last eruptions over a billion years ago), and no global magnetic field today.
Why is the Earth–Moon size ratio considered unusual for a planet–moon system?
Show answer
The Moon is ~1/4 Earth's diameter and ~1/81 its mass — huge for a moon. Other moons are tiny relative to their planets (Jupiter's largest is ~1/10,000 of Jupiter's mass). This suggests a giant-impact origin, not ordinary accretion.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- synchronous rotation
- Rotation period equals orbital period, so the same face points toward the planet.
- tidal locking
- Gravity brakes a body's spin until it rotates synchronously.
- escape velocity
- The speed needed to escape a body's gravity permanently.
- exosphere
- An extremely thin outer atmosphere, closer to a vacuum than to air.
- density
- Mass per unit volume (g/cm³).
- barycenter
- The common center of mass about which two bodies orbit.
- near side / far side
- The hemisphere facing Earth / the one facing away.
- moonquake
- A weak "moon earthquake," mostly from tides and impacts.
- regolith
- The loose, pulverized rock layer covering the lunar surface.
Sources & references
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